providers lock: include test files when loading configuration

[liamcervante]

This PR updates the providers lock command so that it also includes providers embedded in Terraform test modules. This makes providers lock match the behaviour of init in terms of provider discovery, and makes sense to me in terms of discovering provider requirements.

I've marked this as a backport into 1.14.1 as I do think it is fixing a bug, but it does also necessarily introduce a new flag to providers lock so maybe that's enough to mean this should be delayed to 1.15.0. I'm happy to take that feedback if people think so.

Fixes #37841

Target Release

1.14.1

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[dsa0x]

I do think it makes more sense to backport this into 1.14. Besides being a bug or not, the additional flag could also be seen as an enhancement, and that is okay to go into a patch version.